1. Field of the Invention
The present invention relates to a method of joining dissimilar metal plates of an aluminum alloy plate and a plated steel plate by means of adhesion of an adhesive and spot welding, and the like.
2. Description of the Background Art
Conventionally, in the fields of transport equipment such as automobiles and rail cars, machine parts, architectural buildings and the like, research and development concerning methods of joining dissimilar metals such as, for example, a steel material and aluminum alloy, and a joined body of such dissimilar metals.
In particular, in the field of automobiles, there is further demand of achieving the weight-saving of the vehicle body in light of the severe energy situation in recent years, and light-weight aluminum alloy plate materials are often used together with steel plates. Nevertheless, with a product such as an automobile that is used under a severe corrosion environment, if the steel plate and the aluminum alloy plate are directly joined, there is a problem in that corrosion (electrolytic corrosion) caused by the potential difference between the two occurs easily. Thus, as technology of preventing such electrolytic corrosion, the so-called weld-bond method of joining both of the metal plates via an adhesive and performing spot welding thereto is known.
However, since an adhesive has inferior conductivity and will cause an increase in the conduction resistance during the spot welding process, it is not possible to increase the welding current sufficiently. Moreover, since the cleanliness factor of the interface of the steel plate and the aluminum alloy plate is also aggravated, there is a problem in that, consequently, sufficient joint strength cannot be obtained.
Thus, for example, Japanese Unexamined Patent Application Publication No. 2008-23583 describes that, upon joining two metal plates of dissimilar metals based on weld-bonding, both of the metal plates are caused to come in direct contact by discharging the adhesive (sealing material) interposed between both of the metal plates at least from the center of the joint part, and passes a welding current therethough in the foregoing state.
Moreover, as described in Japanese Unexamined Patent Application Publication No. S59-193773, also known is a joining method of including a thermoplastic resin spacer in the adhesive layer applied between two metal plates, pressurizing and passing currently through both of the metal plates in the foregoing state with the electrodes for welding, and performing welding while removing the adhesive and the spacer from the joint part.
Meanwhile, Japanese Unexamined Patent Application Publication No. 2008-23583 forms the tip of the electrode in a convex curved surface in order to efficiently discharge the adhesive from the center of the joint part. However, if the tip of the electrode is formed in a convex curved surface, although the adhesive of the portion corresponding to the apex of such convex curved surface will be discharged reliably, the adhesive will only be discharged locally, and, as a result of the joint area of the steel plate and the aluminum alloy plate becoming small, there is a possibility that sufficient joint strength cannot be obtained. Moreover, although Japanese Unexamined Patent Application Publication No. 2008-23583 also proposes heating the vicinity of the joint part using an external heating means configured from a high frequency coil or the like in order to facilitate the discharge of the adhesive, there is a possibility that the temperature of the adhesive of the joint part may not be effectively increased due to the influence of the thermal conduction to the periphery thereof.
Since Japanese Unexamined Patent Application Publication No. S59-193773 performs pressurization and conduction in a state of clamping a thermoplastic resin spacer between the metal plates, it is expected that the adhesive can be effectively discharged from the joint part in comparison to Japanese Unexamined Patent Application Publication No. 2008-23583. Nevertheless, if the metal plates are joined in a state where such a thermoplastic resin spacer is interposed therebetween, there is a possibility that the thermoplastic resin spacer will melt and flow to the outside, for example, when the obtained joined body is coated and subject to thermal treatment in the subsequent drying step. If resin flows out as described above, the sealability of the joint part will be lost, and, in particular, this will become a crucial defect as a structural material of a vehicle body. Moreover, Japanese Unexamined Patent Application Publication No. S59-193773 does not describe the preferred conditions during pressurization and conduction, and it was not necessarily clear as to how the process from pressurization to conduction needs to be performed in order to obtain sufficient joint strength.